1. Field of the Invention
The present invention relates to a power supply circuit, and more particularly, to a power supply circuit comprising a reference voltage generating circuit and an internal voltage generating circuit.
2. Description of Related Art
Conventionally, a power supply circuit for a semiconductor device, such as a memory, is constituted by a circuit as illustrated in FIG. 4. FIG. 4 is a circuit diagram schematically showing the composition of a power supply circuit. Referring to FIG. 4, a power supply circuit 10 principally comprises a reference voltage generating circuit 11 and an internal voltage generating circuit 13.
The reference voltage generating circuit 11 comprises a resistor R, one end 15 of which is coupled to a power supply terminal Vcc, and a first N-type field effect transistor (NMOSFET) (Tr1), of which the drain electrode 19 is coupled to the other end 17 of the resistor R, the source electrode 21 is coupled to an earth terminal Vss, and the gate electrode 23 is coupled to the drain electrode 19.
Furthermore, the internal voltage generating circuit 13 comprises a second NMOSFET (Tr2), of which the gate electrode 25 is coupled to the drain electrode 19 of the first NMOSFET (Tr1) and the source electrode 27 is coupled to an earth or ground terminal Vss, and a constant voltage generating circuit 31, which outputs a constant voltage, coupled between the drain electrode 29 of the second NMOSFET (Tr2) and a power supply terminal Vcc (FIG. 4).
The output voltage (internal voltage) taken from the output terminal 33 of the constant voltage generating circuit 31 forms a power voltage for driving later stage circuits which are connected to the internal voltage generating circuit 13.
In this power supply circuit 10, the voltage between the power supply terminal Vcc and the earth terminal Vss is divided by the resistor R and the first NMOSFET (Tr1), and this divided voltage forms an output of the reference voltage generating circuit 11 (called a reference voltage). An output point is the connection point N between the gate electrode 23 and drain electrode 19 of the first NMOSFET (Tr1). Moreover, the resistance value R is set such that the voltage between the connection point N and the earth terminal Vss is substantially the same as, or slightly higher than, the threshold voltage of the first NMOSFET (Tr1). Furthermore, the first and second NMOSFETs (Tr1 and Tr2) are transistors which are manufactured under the same conditions and have the same effective composition and operation. Therefore, the threshold voltage of the first NMOSFET (Tr1) and the threshold voltage of the second NMOSFET (Tr2) have effectively the same value.
Next, the operation of the power supply circuit 10 is described.
Firstly, in the reference voltage generating circuit 11, the divided voltage obtained by the resistor R and the first NMOSFET (Tr1) is applied constantly to the gate electrode 23 of the first NMOSFET (Tr1). Since this voltage has a value equal to or higher than the threshold voltage of the first NMOSFET (Tr1), the first NMOSFET (Tr1) is constantly switched on. Even if there is a change in the voltage of the power supply terminal Vcc, a constant reference voltage signal is output from output point N. The reference voltage signal from the output point N is applied to the gate electrode 25 of the second NMOSFET (Tr2) of the internal voltage generating circuit 13. Since the reference voltage applied to the gate electrode 25 of the second NMOSFET (Tr2) is equal to or higher than the threshold voltage of the second NMOSFET (Tr2), the second NMOSFET (Tr2) is switched on (an ON state), and the constant voltage generating circuit 31 coupled between the drain electrode 29 of the second NMOSFET (Tr2) and power supply terminal Vcc is driven. The constant voltage generating circuit 31 is driven when the second NMOSFET (Tr2) is switched on. Therefore, the output signal from the constant voltage generating circuit 31 can be outputted as an internal voltage signal.
The first and second NMOSFETs (Tr1 and Tr2) of the power supply circuit described above are transistors which are formed by the same process and have substantially the same composition and operation.
With the miniaturization of semiconductor devices in recent years, transistor gate lengths have been becoming shorter. Usually, the gate length in a transistor used in a generic memory is approximately 2-3 .mu.m. If the first and second NMOSFETs (Tr1 and Tr2) are manufactured by the same process, then there is a risk that the threshold voltage of the second NMOSFET (Tr2) will be higher than the threshold voltage of the first NMOSFET (Tr1), due to variations during the manufacturing process. If the threshold voltage of the first NMOSFET (Tr1) is indeedhigher than the threshold voltage of the second NMOSFET (Tr2), then the voltage input from the reference voltage circuit 11 to the gate electrode 25 of the second NMOSFET (Tr2) in the internal voltage generating circuit 13, in other words, the reference voltage, which is set to virtually the same level as the threshold voltage of the first NMOSFET (Tr1), will ultimately have a lower value than the threshold voltage of second NMOSFET (Tr2). Therefore, even if the reference voltage is applied to the gate electrode 25 of the second NMOSFET (Tr2), the second NMOSFET (Tr2) will not switch on and the constant voltage generating circuit 31 will not be driven. Consequently, a problem arises in that a stable internal voltage cannot be obtained.
Therefore, one method conceived for switching on the second NMOSFET (Tr2) involves setting the voltage between the output point N and earth terminal Vss to a higher value than the threshold voltage of the first NMOSFET (Tr1) by lowering the resistance value of the resistor R. However, if this method is adopted, the power consumption by the reference voltage generating circuit 11 will become greater than in the prior art (first problem).
Accordingly, the development of a power supply circuit for miniaturized semiconductor devices, whereby a stable internal voltage is obtained without increasing the power consumption of the reference voltage generating circuit, has been awaited.
Moreover, in the circuit connected after the power supply circuit 10 (hereinafter, called the later or second stage circuit), if the output from the power supply circuit 10 is used as the power supply voltage of the second stage circuit, then the output voltage (internal voltage) from the power supply circuit 10 is applied constantly to the second stage circuit. This state is then taken as the standby state of the second stage circuit. To drive the second stage circuit, an active signal is applied to the second stage circuit and when this active signal is applied, the second stage circuit is driven and a large current flows through the second stage circuit.
Furthermore, the earth terminal of the second stage circuit is connected to the earth terminal Vss. Therefore, if a large current flows in the second stage circuit whilst it is being driven, the electric potential of the earth terminal Vss will increase and there is a possibility that the threshold voltage of the second NMOSFET (Tr2) in the internal voltage generating circuit of the power supply circuit will rise temporarily. Consequently, a problem may arise in that the second NMOSFET (Tr2) will not switch on even if the reference voltage is applied to the gate electrode 25 of the second NMOSFET (Tr2), and hence a stable internal voltage cannot be obtained (second problem).
Therefore, the development of a power supply circuit whereby a stable internal voltage is supplied, even when the second stage circuit using the output of the power supply circuit as a power supply voltage is driven, has been awaited.